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Hi Art,
The others have commented on my original response about (remote possibility of) a slipping clutch (re: "... (unless your clutch is literally melting :-)....) so we'll leave that.
But I also offered that maybe, "...Only some electronic defect developing in your '93's speedometer circuit board might possibly explain this...." So, since you're the electronics guru on this forum, can you think of anything on that circuit board that could slightly change the indicated rpm? I have no idea how the circuit changes the pulse frequency from the rear axle into an analog speedometer reading -- but could some capacitor or resistor that's failing slightly change the reading? Something that would alter the calibration, if it's possible?
I once (for fun) "invented" a crude tach for my garden tractor -- it took pulses (via a resistor) from a wire wrapped around the spark plug lead and simply charged up a capacitor that drained through an analog meter and a resistor: the higher the pulse frequency, the higher the voltage in the capacitor and the greater the current flowing through the meter. It took a lot of adjustment of the resistor and some trial and error finding a suitable capacitor to come close to the right speed, and it wasn't a good correlation over the rpm range, but I had some fun doing it. Anything like that here, assuming real engineers could make it work better than I could :-) ?
[BTW, that was years before a cheap little digital/LCD tach for single cylinder engines started showing up in tractor parts counters. :-) But having grown up building HeathKit projects (anyone else old enough to remember those?), I still like to dabble. Too bad about Radio Shack, though -- they used to be useful -- but thankfully I've still got a real electronics component store near me to satisfy my urges :-).]
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